Several fiber-optical acoustic sensors have been developed in the recent past. These operate according to one of the following principles.
Two single-mode optical fibers are arranged in the form of an interferometer in which a length of one of the fibers is subjected to a magnetic or acoustic pressure field and forms the sensing arm. The other fiber is shielded from the field and forms the reference arm. Then, by the photoelastic effect, a phase change is induced in the light propagating in the sensing fiber. Recombining the light from the sensing arm with that from the reference arm results in interference fringes which give a measure of the magnitude of the magnetic field or the magnitude of the acoustic wave. The two-fiber interferometer arrangement is very sensitive to changes in environmental conditions, such as temperature, pressure, air currents, for example, which also introduce phase changes in the propagating light. Because the two fiber arms are physically separate, differential environment conditions face each and seriously affect the interferometer stability. As well, the state of polarization (SOP) of the light emerging from each fiber arm must be correct (and remain so) or the two will not completely interfere. Currently available single-mode fibers cannot maintain a specified SOP and, as the SOP in the fibers change, fringe visibility may fall to zero. U.S. Pat. No. 4,162,397 issued July 24, 1979 to Joseph A. Bucaro et al. for Fiber Optic Acoustic Sensor discloses a two fiber acoustic sensor wherein acoustic waves incident on the coil changes its index of refraction at the region of incidence. The index change causes a phase shift in the transmitted light which is detectable to denote the presence of sound waves.
The above interferometer can be arranged such that both the light paths propagate within the same fiber which may be either multimode or support only a few modes. In this case, the field condition changes the phase of all the propagating modes which interfere to produce a complex interference pattern at the fiber output. Probing this pattern with a suitable aperatured detector gives a signal proportional to the magnitude of the magnetic or acoustic field condition, but the approach is wasteful of light as only a portion of the transmitted light can be utilized. If selective excitation at the input is used to excite only two modes of the fiber, then mode conversions due to imperfections can lead to problems. The single fiber inerferometer has one advantage in that it does not require beamsplitting devices.
Another principle is disclosed in U.S. Pat. No. 4,342,907. When a light carrying fiber is bent, some of the guided light is leaked or radiated therefrom. A light carrying fiber is arranged to lie between two corrugated plates which, when subjected to an acoustic wave, bend (strain) the fiber in induce light losses therefrom to modulate the transmitted light in proportion to the magnitude of the acoustic wave signal. With this approach, problems arise when the surrounding pressure changes significantly.
U.S. Pat. No. 4,173,412 relates to a sensor based upon the measurement of strain induced in an optical fiber by stressing it perpendicular to its lontigudinal axis. The fiber has no initial birefringence or any means of introducing it. The invention described herein discloses details including the introduction of large birefringence in the fiber by winding it under tension on a small diameter cylinder or mandrel so as to introduce large initial birefringence. This large birefringence introduced by the tension coiling is essential for the sensor to operate stably.